Ground engaging tools, such as bucket teeth, ripper tips, track shoes, and other parts for construction machines operating in soil and rock, require a combination of high hardness throughout the tool to resist wear, high fracture toughness to avoid excessive tool breakage, and sufficient temper resistance to prevent loss of hardness during operation at elevated temperatures. A number of attempts have heretofore been made to provide a steel material having all of these characteristics.
A number of steel materials proposed for use in applications requiring a combination of desirable hardenability, toughness, and temper resistance properties, have compositions which include relatively high amounts, i.e. above 3% of chromium. For example, a steel mainly intended for use as an excavating tool edge material for construction machines is described in U.S. Pat. No. 3,973,951 issued Aug. 10, 1976 to K. Satsumabayashi et. al. This steel has a chromium content of 3.0% to 6.0%. Similarly, a wear resisting steel developed for use as a ripper tip and having 3.0% to 5.0% chromium is described in Japanese Patent 54-42812 issued Dec. 17, 1979 to applicant Kabushiki Kaisha Komatsu Seisakusho. Another steel intended for use in mining buckets and other mineral processing operations, and having a composition that preferably includes 3% to 4.5% chromium is described in U.S. Pat. No. 4,170,497 issued Oct. 9, 1979 to G. Thomas et al. The steel material embodying the present invention has high hardenability, toughness, and temper resistance, but contains no more than 2.0% chromium, and preferably between 0.35% and 1.25% chromium.
Other steels intended for use in applications requiring a combination of high hardenability and toughness require significant amounts of nickel. Examples of these compositions are disclosed in U.S. Pat. No. 2,791,500 issued May 7, 1957 to F. Foley et al, U.S. Pat. No. 3,165,402 issued Jan. 12, 1965 to W. Finkl et al, U.S. Pat. No. 3,379,582 issued Apr. 23, 1968 to H. Dickenson and, more recently, U.S. Pat. No. 4,765,849 issued Aug. 23, 1988 to W. Roberts. The steel embodying the present invention does not require the presence of nickel to achieve the desired hardenability and toughness properties.
The above mentioned U.S. Pat. No. 4,765,849 teaches the inclusion of aluminum and titanium in the steel composition, similar to that proposed by the present invention. However, U.S. Pat. No. 4,765,849 adds substantially higher amounts of aluminum (o.4% to 1.0%) than that specified in the present invention, to intentionally form aluminum nitride in the solidified product.
Contrary to the teaching of the U.S. Pat. No. 4,765,849, it is generally recognized that the presence of aluminum nitride is undesirable in steel requiring high hardenability and toughness. For example, U.S. Pat. No. 3,254,991 issued Jan. 7, 1966 to J. Shimmin, Jr. et al and U.S. Pat. No. 4,129,442 issued Dec. 12, 1978 to K. Horiuchi et al specifically exclude aluminum from the composition to prevent the formation of aluminum nitrides.
U.S. Pat. No. 5,131,965 issued Jul. 21, 1992 to J. McVicker and assigned to the same company as this instant invention, discloses a steel having high hardenability and toughness. However, U.S. Pat. No. 5,131,965 uses higher chromium to attain high hardenability and temper resistance without exploiting the hardenability and precipitation effect of boron to obtain high fracture toughness, as is done in the present invention. In addition, the present invention uses boron to lower grain boundary energy and, thus, improve fracture toughness.
The present invention is directed to overcome one or more of the problems as set forth above.